This project is aimed at understanding the molecular basis of intracellular iron metabolism. The components of a post-transcriptional cis-trans regulatory model have been identified and characterized in previous years in this laboratory. Iron-responsive elements (IREs) are RNA stem-loops found in the 5' end of ferritin mRNA and the 3' end of transferrin receptor mRNA. We have cloned and expressed an iron-sensing protein, the iron- responsive element binding protein (IRE-BP) that binds IREs when iron levels are depleted, resulting in the translational inhibition of ferritin and stabilization of the mRNA half-life of the transferrin receptor. The IRE-BP is 30% identical in sequence to aconitase, a mitochondrial Krebs cycle enzyme containing cluster. The IRE-BP has aconitase activity, and in vitro manipulations of iron result in changes in RNA binding. A complete description of this "iron switch" and the role of aconitase enzymatic activity in the function of the IRE-BP will lead to a more complete understanding of this complex RNA-protein regulatory interaction.